I. Field of the Invention
The present invention relates to a method for forming a shell mold for a foundry casting operation with an improved shrink bob portion.
II. Description of the Prior Art
Essentially the previously known shell molding processes use a thermosetting molding material to form rigid molds having high gas permeability, good surface smoothness and dimensional stability. The molding material, which is generally a dry mixture of silica or other refractory oxide sand and a minor amount of plastic or resin binder, is normally used in powdered form with no water being added.
The shell mold is prepared by applying the dry mixture of the molding material to a hot casting pattern for a relatively short period of time. A generally uniform layer of the molding material adheres to the pattern surfaces due to the melting resin which in turn bonds the sand together to accurately reproduce the details from the casting pattern. The casting pattern typically forms one-half of the shell mold so that by fixing together two halves of a shell mold, a complete casting mold is achieved. Molten metal is thereafter poured into the shell mold through a sprue to form a casting.
As is well known in the art, horizontally elongated or intricate casting patterns are prone to surface defects after the metal casting operation. These surface defects in the cast metal are not only undesirable but oftentimes result in the scrapping of the metal part.
In order to minimize surface defects during the metal casting operation, previously known shell molds have been provided with vertically extending shrink bob chambers or risers. The shrink bobs are filled with the molten casting metal during the casting operation so that each shrink bob forms a hydrostatic or pressure head which exerts a downward pressure upon the molten metal. This pressure head aids in feeding the molten casting metal to all portions in the shell mold and absorbs metal shrinkage which would otherwise occur without the shrink bob.
These previously known shrink bobs are necessarily conical or frusto-conical in shape with the base of the conical portion open to the casting chamber. This construction of the shrink bob is necessary since otherwise the shell mold could not be removed from the casting pattern after the mold had hardened onto the pattern. These previously known shrink bobs are thus inefficient in that the shrink bob must be vertically elongated in order to generate a sufficiently large hydrostatic pressure necessary to aid in feeding the molten casting metal. Moreover, due to the conical shape of the previously known shrink bobs the incremental increase in hydrostatic pressure decreases proportionately with the height of the shrink bob.
Removal of the shrink bob by machining is necessary for the casting to form a finished product. Since the base of the frusto-conical shrink bob is quite large, removal by machining is difficult and expensive.